Polymer electrolyte membrane fuel cell assemblies are relatively low temperature low operating pressure fuel cell assemblies that utilize a catalyzed polymer membrane electrolyte to process air and a hydrogen-rich fuel, or pure hydrogen, to produce electricity and water. PEM fuel cells are well suited for use in mobile applications such as automobiles, buses, and the like, because they are relatively compact, light in weight and can operate at essentially ambient pressure. They also have utility in stationary applications. The membrane in fuel cells of this type must be kept moist during operation of the fuel cells lest they dry out, and they also require that excess product water formed during the reaction be removed from the cells lest they flood.
One type of fuel cell system which uses solid reactant flow field plates with integral reactant flow field plates can be cooled by spraying water droplets into the reactant flow streams before they enter the cells. The resultant moisture in the reactant streams will evaporate in the cells and will thus cool the cells during operation of the power plant. The reactant streams will also sweep out product water from the cells so as to protect them from flooding. This cooling and water removal approach requires the inclusion of adjunct equipment for spraying the water droplets into the reactant streams, and also involves the inclusion of water impermeable reactant fluid flow plates on both the anode and cathode sides of the fuel cells so as to ensure that product water will be swept out of the cells by the exiting reactant fluid flow streams. This type of system also requires relatively high pressure drops to maintain the gas phase velocities required to entrain liquid water droplets in the flow. These high pressure drops in turn increase parasitic loads and lower system efficiency. Furthermore, imprecise control over local humidity levels can subject the membrane to mechanical stress and accelerate membrane failure. This type of system is typically purged of water during shutdown in freezing ambient conditions by purging the system with a dry gas until a substantial portion of the water remaining in the system has been evaporated and removed from the system. This method of preventing the formation of ice in the system during freezing conditions is not satisfactory since it results in a substantial dry out of the membrane which severely limits performance of the cells on subsequent start, until the membrane is hydrated. Repeated use of this solution to the freeze problem will ultimately result in membrane degradation over time, because the membrane will degrade with humidity cycling.
Alternatively, this type of fuel cell system can utilize two porous plates. In this configuration, the porous anode and cathode separator plates serve to humidify the reactants, and the porous cathode plate removes product water from the electrochemical reaction. Under freezing conditions, when utilizing porous cathode and anode reactant flow field plates, the plates will not be purged of water, thus the water in the plates will freeze in situ after shutdown of the system. This eliminates the need for a long, energy-intensive purge and eliminates forced membrane humidity cycles which can deteriorate the membrane. Additionally, with a system using two porous plates, the internal resistance of the cells on restart is relatively low, meaning that high power can be drawn from the cells immediately upon restart. The disadvantage with this type of system is that it is designed to work with two porous plates in each cell in the stack, both of which contain frozen water upon restart, and therefore, it requires significant time and energy to thaw the ice in both plates. During the time when the internal cell water (ice) is thawing, there is no efficient way for removing product water from its point of generation in the cathode catalyst layer. The accumulation of water in the cathode catalyst layer and the adjacent gas diffusion layers will restrict gas access and thus reduce the maximum rate of power generation until the ice is thawed and a means of water removal is reestablished. Once the ice thaws and the temperature of the cells climbs, full power can then be rapidly achieved.
One solution to the problem of large thermal mass (ice) in the cells during freezing shutdown involves evacuating the porous plates when the power plant is shut down so that water in the plates and the coolant channels in the plates will be sucked out of them and thus no ice will form in the plates during shut down under freezing conditions. This is not a satisfactory solution to the problem, as it requires adjunct equipment to create the shut down vacuum in the power plant and it also provides no way for isolating reactant gases during shut down until the plates are refilled with water.
It would be highly desirable to have a solution to the problem which would have the advantages of the porous plate system but with much lower thermal mass and no reactant gas crossover during operation or shutdown.